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A subway train derailed Tuesday deep below Moscow's streets, twisting and mangling crowded rail cars at the height of the morning rush hour. At least 21 people were killed, Russian officials said, and 136 were hospitalized, many with serious injuries.

Russia’s Moscow Metro is world-famous for its palatial interiors with mosaics, chandeliers and marble benches. Park Pobedy, where the derailment occurred, is Moscow's deepest metro station — 275 feet below the surface, which made the rescue particularly difficult.

It was unclear what caused the train to derail. Lines of inquiry included a fault in one of the cars or the sinking of the roadbed, according to Vladimir Markin, spokesman for Russia's top investigative body. He said earlier reports that a power surge triggered an alarm, causing the train to stop abruptly, were incorrect.

Of the 136 people hospitalized, at least 42 were in grave condition, health officials said. One citizen of China and another of Tajikistan were among those killed, Russian news agencies quoted city officials as saying.

Over 1,100 people were evacuated from the train, which was stuck between two stations, in a rescue operation that ended more than 12 hours after the accident. One woman taken from the scene died at a Moscow hospital.

Injured, bloodied and bandaged passengers were carried on stretchers out of metro stations, and helicopters flew the most seriously hurt to hospitals. Passengers looked stunned or were crying after being helped to the surface by emergency services.

Moscow's Deputy Mayor Peter Biryukov said three bodies were recovered from the wreckage but that others remained underground.

"There is no one alive left," Biryukov said "The cause is not known. The work continues." A city transport services spokesman told news agency Interfax that all passengers were evacuated from the affected stations by midday, dismissing reports that some passengers were still trapped in the underground tunnel.

"It braked very hard. The lights went off, and there was lots of smoke," a man with a bloodied nose told Rossiya-24 state television. "We were trapped and got out only by some miracle. I thought it was the end. Many people were hurt, mostly in the first rail car because the cars ran into each other."

The Moscow metro is the world's busiest, with as many as 9 million people on weekdays riding a system that is widely recognized for its reliability. The underground network has expanded from 13 stations in 1935 to 194 stations across the megalopolis today.

In video released by the Emergency Situations Ministry, several wrecked train cars looked almost coiled, occupying the entire width of the tunnel. Workers were trying to force open the mangled doors of one car to retrieve bodies. Photos posted on social media sites showed passengers walking along the tracks in the dimly lit tunnel.

Russian officials rushed to open an investigation into the accident. President Vladimir Putin, who is traveling in Brazil, demanded a detailed investigation into the "reasons for the event" and asked the country's top investigators to open a criminal case, his spokesman told Russian news agencies.

Mayor Sergei Sobyanin also told reporters that unnamed officials will not only be fired but also charged with crimes, though he would not say what charges they might face. Prime Minister Dmitry Medvedev offered his condolences for the victims and their families.

While technical glitches are regular occurrences in the Moscow Metro, the subway hasn't seen deadly accidents in decades.